MOLECULAR-WEIGHT DETERMINATION FROM LIGHT-SCATTERING AND REFRACTION IN SOLUTIONS - A NEW AND COHERENT THEORETICAL EQUATION

The classical theoretical equation allowing for the determination of t he solute's molecular weight (M(2)) from light scattering (LS) and ref ractive measurements in diluted solutions is reexamined. Serious theor etical and practical defects in its derivation and application are poi nted out, especially about the refraction increment dn/dc. A new theor etical equation is deduced from a more consistent application of the b asic molecular orientation formulae to binary liquid mixtures. More ad equate additive molecular properties, the specific mean polarizability <(alpha)over bar>/M and the specific square of the mean polarizabilit y <(alpha)over bar>)(2)/M instead of <(alpha)over bar> and (<(alpha)ov er bar>(2), are considered with respect to refraction and LS, respecti vely. Inadequate approximations are avoided and a corrected LS intensi ty expression is used. The new equation is tested upon several typical mixtures of small molecules from measurements either performed in thi s work or collected in the literature (for LS perpendicular to the inc ident beam) to ensure objectivity. Examples of measurements in macromo lecule solutions from other authors are also analysed. The new equatio n is suitable for practical applications, because it is based upon eas y relative measurements only, and allows for a reliable determination of the M(2) values in macromolecules. The reasons for which the classi c equation leads to realistic values, in spite of its theoretical inco nsistency, are also explained.